The normal cardiac cycle includes contractions of the atrial muscles, which are activated by the autonomic sinoatrial node (SA node), also called the sinus node. The electrophysiologic (EP) signal generated by the SA node spreads in the right and left atrium leading to their contraction. The EP signal further reaches the atrioventricular node (AV node) situated between the atria and the ventricles. The AV node delays the EP signal, giving the atria time to contract completely before the ventricles are stimulated. After the delay in the AV node the EP signal spreads to the ventricles via the fibers of the His-Purkinje system leading to the contraction of the ventricles. After the contraction the atria are relaxed and filled by blood coming from venous return. The entire cardiac cycle is the combination of atrial and ventricular contraction, i.e. depolarization, and their relaxation, i.e. repolarization.
As is known, the cardiac cycle can be measured non-invasively by attaching small electrodes on the skin of the patient. The voltage differences caused by the heart between the electrodes are measured and recorded in order to obtain the electrocardiogram (ECG) of the patient.
In this connection, reference is made to FIG. 1, which shows one cycle of an ECG signal. As is commonly known, and also shown in the figure, the waves of the ECG signal (i.e. the depolarisation and repolarisation events in the heart) are named alphabetically from P to U. The ECG signal shows each phase of the cardiac cycle: the P wave represents the systole of the atria, the QRS wave represents the systole of ventricles, and T wave represents their repolarization. Modern ECG devices use digital signal processing to analyze the shape and the consistency of, and the durations between these waveforms.
The heart rate (HR) can be measured by calculating the number of QRS waves in a minute. The HR may be expressed as a minute rate or as beats per minute (bpm). The rate of a heart functioning in a normal manner is not a constant, and the variation of the rate, which is commonly called the heart rate variability (HRV), has become one of the widely used markers for indicating the cardiac condition of a patient.
The ECG signal is thus analyzed for detecting various heart disorders, such as abnormalities in the heart rhythm, also termed arrhythmias. A common abnormal cardiac rhythm is the atrial fibrillation (AF). In the AF, the atria lose the nominal control pattern induced by the SA node. The contraction of the atria may start simultaneously in several points of the atria, spreading spuriously in the atria. Furthermore, the contraction is chaotic and the P waves disappear from the ECG signal. The contraction frequency of the atria during an AF episode is typically between 200 and 400/min. The AV node is not able to correct the lost sinus rhythm and the ventricular contractions are irregular. The ventricular beat-to-beat variation is high and can be seen as quick changes in the intervals between consecutive QRS complexes of the ECG signal.
The risk of an AF episode increases with age, and the occurrence of AF episodes is common among elderly people. However, the AF is not an immediate life-threatening disease. Yet, if the AF episodes remain persistent or last long, the blood remains circulating in the atria and may start to congeal. The resulting small blood clots may find their way to the brain and develop a stroke. Thus it is necessary to diagnose the occurrence and duration of the AF episodes in order to initiate a treatment and to follow the outcome of the treatment.
According to the American Heart Association (AHA) the prevalence in the USA of the AF and atrial flutter is about 2,000,000, while the total annual mortality caused by the said heart abnormalities is about 67,000. The listed hospital discharges are close to 400,000 (AHA, Heart Disease and Stroke Statistics—2003 Update). Atrial arrhythmia represents more than 50% of all hospital discharges of all types of cardiac arrhythmias. In the year 2002, 2.1 billion USD was paid to Medicare beneficiaries in this disease group. About 70% of people with atrial fibrillation are between 65 and 85 years and about 15% of strokes occur in people with atrial fibrillation (AHA).
The treatment of atrial arrhythmia may require a cardioversion to return the heart to a normal rhythm. This is carried out by medication or defibrillation, and a pacemaker may also be used. In order to make the diagnosis and to monitor the outcome of the treatment in acute care, the ECG signal obtained from a bedside monitor is commonly utilized. An esophagus ECG may also be used, in which a specific electrode setting is placed into the patient's esophagus close to the heart. This measurement method may give a better signal than a normal ECG measurement for identifying the atrial contraction, i.e. the P waves. This, in turn, may ease the rhythm diagnosis.
To maintain the sinus rhythm after the cardioversion, anti-arrhythmic medication may be used. However, this may cause side-effects such as pro-longed QT duration, which may be life-threatening. This is one of the reasons why continuous ECG monitoring is a common procedure for hospitalized arrhythmia patients and a minimum requirement in acute care of such patients.
The ECG arrhythmia algorithms of the state-of-art bedside monitors and telemetry systems feature an extensive analysis of ventricular beats, but not atrial analysis. The ECG algorithms nominally label the atrial arrhythmia or related atrial disorders for example by “Irregular Rhythm”, which is not an exact diagnostic statement in the way “Atrial Fibrillation” would be. Therefore, such algorithms are not useful in acute care where the occurrence and recurrence of AF episodes should be monitored.
Although the current bedside monitors do not feature atrial analysis, methods and devices have been disclosed for detection of atrial fibrillation. These methods are, however, diagnosis tools which aim to classify the type of the AF and to give information for the selection of the most appropriate treatment. Thus their usage in acute care is limited. Such methods are disclosed in U.S. Pat. No. 6,178,347 (Olsson) or in U.S. Pat. No. 6,064,906 (Langberg et al.). In these methods, the ventricular component, i.e. the QRS wave, is removed from the ECG signal and a frequency analysis of the remaining signal, indicative of the atrial activity, is performed for classifying the type of atrial fibrillation in order to produce a prognosis of the patient.
Given the high prevalence of atrial arrhythmia and the importance of the prompt initiation and continuous monitoring of the treatment, the need for an automatic and precise diagnosis of atrial arrhythmia is therefore evident in acute care. The present invention seeks to provide a mechanism for this purpose.